This invention relates in general to apparatus and methods for the stabilization of sludge such as sludge formed during the treatment of municipal waste water. More specifically, it relates to an apparatus and method where the sludge is intensively mixed with microscopic bubbles of oxygen and ozone in one or more reaction tanks each having an in-built mixer.
The disposal of sludge produced by waste water treatment plants is an increasingly difficult problem. In recent years the problem is intensifying on the supply side as the population grows and there is an increased emphasis on the treatment of waste water to meet pollution control standards. On the disposal side, landfill sites are becoming less available, or available only if the sludge is transported over longer distances at an increased cost. Heretofore, a principal disposal technique was ocean dumping. However, ocean dumping has been found to have an adverse environmental impact and is scheduled to be totally phased out, at least in the United States, in the near future.
Other known disposal techniques for sludge include burning it, particularly at large centralized incinerators, and using it as a fertilizer or a mulch for agricultural applications. The burning of sludge at present produces a variety of air pollution problems, particularly since sludge typically contains high concentrations of heavy metals. The heavy metal content and the presence of harmful bacteria and other organic matter seriously limits the use of sludge for agricultural purposes. In addition, conventional treatment methods result in the sludge product, even one where heavy metal content is controlled and the sludge is otherwise stabilized, that has a high water content and therefore is heavy and expensive to transport.
To disinfect sludge, it has been known for some time that it may be treated with ozone and/or oxygen. For example, D. Thiirumirthi discloses the application of ozone for waste water treatment in Water and Sewerage Works (1968) at page L-R106. Further, as early as 1971 H. M. Rosen made a presentation at the University of Wisconsin relating to the application of ozone to the treatment of sludge. It is also known to adjust the pH of the sludge to remove heavy metals.
U.S. Pat. Nos. 3,525,685; 3,772,188; and 4,581,137 to R. N. Edwards describe various ways to use oxygen and ozone to treat sewer lines, municipal sewerage, and liquid sludge. U.S. Pat. Nos. 4,464,257 and 4,500,428 to J. M. Lynch disclose a similar system for treating sludge in several hyperbaric reaction vessels with the use of intermediate sludge thickening devices.
U.S. Pat. Nos. 3,772,118 and 4,581,137 to Edwards describe apparatus for treating municipal sewerage with oxygen and ozone gas in a large spherical, stainless steel reactor vessel. The sludge fills the vessel approximately halfway. An oxygen diffuser, an improved version of which is subject of the '137 patent, diffuses the oxygen and ozone gases into the liquid sewerage at the bottom of the reactor vessel. To interact the sewerage and the gas, the liquid sewerage is directed by a pump and pipes to the upper end of the vessel where it is discharged against a rotating commutator that breaks up the sewerage into droplets which are deflected downwardly through the upper half of the vessel toward a pool of sewerage held in its bottom half. The sewerage interacts with the oxygen and ozone gases that fill the upper half of the vessel. The droplet to gas contact surface area and reaction time are not conducive to a rapid and complete stabilization of the sludge, e.g. in one spraying. The gas also interacts with the sewerage through direct diffusion into the sewerage pool because the diffuser is submerged within the sewerage at the bottom of the vessel, but this additional gas-sludge interaction does not, in practice, sufficiently enhance the performance characteristics of the Edwards type apparatus to a level that is comparable with that achievable with the present invention.
In U.S. Pat. Nos. 4,464,257 and 4,500,428 to Lynch et al., the sewerage interacts with oxygen and ozone gas in much the same manner as in Edwards. Lynch et al. use a large closed vessel with a sludge disperser mounted within the vessel that sprays watery sludge into an open part of the vessel filled with ozone and oxygen or air. A pump and recycling piping directs the watery sewerage from the bottom of the vessel to the top where it is then directed to the spray disperser. Lynch also teaches adding chlorine to enhance oxidation, but at a substantial cost disadvantage. The pH is adjusted by adding sulphuric acid to the sludge held in the tank. Lynch also teaches an oxygen diffuser located near the bottom of the reaction vessel at its interior. It is significant to note that the reaction time to achieve stabilization for both Edwards and Lynch is comparatively long. In the '137 patent, Edwards mentions a 90 minute cycle of reaction. Lynch et al. mention residence times of 15-60 minutes in each of two reaction vessels which operate on the sewerage, one after the other.
While both the Edwards and Lynch systems are capable of producing a stabilized sewerage end product, the systems are costly, comparatively slow, operate in a batch mode, should have an attendant to monitor their operation, and require that the sludge or waste sewerage being processed be in a liquid state capable of being sprayed within the reaction vessel. For municipal waste water treatment sludge, being in a liquid phase capable of being sprayed means having a dry solid content of about 1% to 3%.
Therefore a principal object to the present convention is to provide a highly compact sludge stabilization apparatus and method which has a comparatively low capital cost and can be operated fully automatically on a continuous basis to produce a treated sludge product that is not biologically dangerous and has removed from it potentially hazardous heavy metals.
Another principal object of the present invention is to provide a sludge stabilization apparatus and method with the foregoing advantages which can treat sludge in a thickened phase with a dry solid content in excess of 3%.
Yet another principal advantage of the present invention is to provide a sludge stabilization apparatus and method with all the foregoing advantages which has a reaction period which is several times faster than known prior art apparatus and techniques.
A further object of the invention is to provide a system for stabilizing sludge which results in a comparatively inert, heavy metal-free sludge which has a dry solid content of approximately 20% to 40%, with attendant lower weight and lower transportation costs.
A still further object of the present invention is to provide a sludge stabilization apparatus and method which can turn normal municipal waste sludge into products which can be used as a fuel or for agricultural purposes such as fertilizer or mulch.
Still other objects of the present invention are to provide sludge stabilization apparatus and method which utilize reactor tanks that can be approximately 1/4 of the size of reactor tanks in prior art systems capable of handling equivalent volumes of sludge, which utilize conventional piping and which avoid the cost, safety testing, and safety hazards of known prior art sludge stabilization systems which pump the sewerage at high pressures for spraying.